1. Field of the Invention
This invention relates to an in situ method of cleaning contaminated soil that relies on the metabolism of the contaminants by indigenous soil microorganisms. Briefly, the method comprises breaking up contaminated soil, placing the broken soil in a substantially enclosed on-the-ground system where air is circulated, providing oxygen under pressure to the broken soil to promote the oxidation of contaminants to increase the temperature of the broken soil and vaporize any contaminants, extracting any gases or vapors from the soil by means of a low pressure air flow, filtering the gas-containing air to separate clean air from the gases or vapors and returning the decontaminated soil to the environment. The present technology shortens clean-up time from, e.g., about five years to five months. The friction created by the circulation of air of any source of oxygen increases the soil's temperature and aids the metabolism of contaminants and their removal. In addition, microorganisms grow better when air and moisture are provided, and this also increases the degree of contaminant biodegradation. The biodegraded contaminants, in turn, may be loosened up from the soil, and transformed into a gas or vapor for removal.
2. Description of the Background
Increasingly, spills and landfills of toxic and carcinogenic materials occur in the soil. If left in place, many of these contaminants find their way into aquifers or into the food supply, and could become public health hazards.
Prior methods proposed for removal of soil contaminants include excavation followed by incineration, in-situ vitrification, bacterial attack, addition of deactivating chemical additives, radiofrequency heating, etc. Although successful in some applications, these methods may be very expensive costing, e.g., hundreds of dollars per ton of soil being decontaminated, and are not practical if many tons of soil are to be treated.
In U.S. Pat. No. 4,670,634, Bridges et al propose a method for in-situ decontamination of spills and landfills by radiofrequency heating. The soil is heated by radio-frequency energy to a temperature higher than that needed to boil water. This is said to increase the permeability of the soil. A vapor and gas collection and containment barrier is installed above the region of the soil to be decontaminated. The heating is continued by dielectric heating after water has boiled from at least a portion of the region so as to heat the portion to elevated temperatures substantially above the boiling point of water. The material is treated in-situ by pyrolysis, thermally-assisted decomposition, distillation, or oxidation. The materials may also be driven from the region, as by distillation or evaporation and steam drive, and then collected and disposed of by incineration.
U.S. Pat. No. 4,670,634 is a significant advance relative to previous remediation methods. However, this patent also presents various disadvantages. The use of radiofrequency power in the MHz range makes the process hard to control as the soil dries out. It results in less uniform heating because of "hot spots" which are overheated every half wavelength and "cold spots" which are underheated. It results in a loss of efficiency in the generation of radiofrequency power. In addition, it emits electromagnetic noise that can interfere with radio communications. Also, because the process of this prior art patent is operated near atmospheric pressure, it requires cumbersome vapor collection barriers at the surface.
Other methods have been previously used to remove industrial pollutants from the vadose zone of the soil. The most common of these methods is excavation, in which all of the contaminated soil is removed and eventually replaced with fresh earth. While excavation is a relatively simple process, it is not practical when large volumes are involved due to prohibitive cost and time factors.
Another method involves a circulation system for leaching contaminants from the vadose layer into the water table where they are recovered by a water removal well and a pump. This process is shown generally in U.S. Pat. No. 4,167,973. Such processes are not always successful due to the low water solubility of most common industrial pollutants, which results in a lengthy and often costly recovery.
A third method involves the creation of a vacuum within a withdrawal well in the vadose zone. The contaminants are urged towards the withdrawal well by injecting air into the soil at points surrounding the withdrawal well where they are vaporized and collected by vacuum withdrawal. Such method is described in U.S. Pat. Nos. 4,183,407 and 4,593,760. These methods are generally effective in the recovery of some contaminants but do not provide for the satisfactory disposal of most contaminants.
Another method for collecting volatile contaminants is that provided by U.S. Pat. No. 4,730,672. This method utilizes a closed-looped device including one or more withdrawal wells surrounded by multiple air reinjection wells connected by a conduit. The volatilized contaminants are drawn through the withdrawal well(s) to the connecting conduit where it is (are) captured or neutralized. Residual air from the withdrawal well is replaced into the ground through the air injection wells to encourage further contaminants to move toward the withdrawal well(s) for collection.
Still another method is provided by U.S. Pat. Nos. 4,984,594 4,984,594; 4,730,672 and 4,890,673. These patents provide an in situ method for removing contaminants that applies vacuum to the soil under an impermeable flexible sheet while heating the soil with an electric surface heater positioned on the soil surface under the sheet. The heater is permeable to vapors emanating from the soil when heated. A permeable mat separates the heater from the impermeable sheet and provides a conduit for vapors flowing to openings provided in the sheet. The openings are connected to a vacuum manifold that facilitates the collection of the vapors. The region under the impermeable sheet is evacuated with a vacuum pump and the soil contaminants are removed by vaporization, steam distillation and/or thermal decomposition.
Filtration is also a common means for separating solids from liquids, usually involving forcing a slurry or mixture of solids and liquids through filtering materials. A slurry is taken to mean a watery suspension or mixture of insoluble solids in a liquid. It encompasses mixtures or suspensions of solid materials comprising particulate matter, gels, sludges and the like, which are at least partially insoluble in a liquid component comprising water, liquid organic or inorganic materials, and mixtures thereof. Such slurries can contain solids that remain in suspension, as well as those that tend to settle naturally with time. Normally, the filtrate or liquid passes downward through the filter with the solids remaining on the filter for recovery.
In many industries, e.g., the mining, oil, chemicals, wood pulp and agricultural industries, unfiltered waste slurries of liquids and solids are placed in dammed earthen areas or "settling ponds", which are concave depressions in the ground, lined or unlined, used to collect slurries of solids and liquids. Generally, such slurries are collected in settling ponds for separation, disposal or other processing. It is desirable to minimize the area of these settling ponds, to maximize disposal rates of slurries and eventually to reclaim the areas including landfills and the like. However, often these areas never dry nor solidify, since evaporation is slow, and the slurried or flocculated solids never settle sufficiently so that the liquids can be pumped off. Effective means for dewatering such settling ponds without removing the sediment or settled solids are thus desirable.
Still another method is provided by U.S. Pat. No. 3,016,345. This method removes contaminants, including solids and liquids such as water particles, from a hydrocarbon liquid by separating out solid materials and increasing the static electrical charges on the water particles, then neutralizing the electrical charges and coalescing the water particles to propagate their size and separating the water from the hydrocarbon liquid.
Still another method of separating liquids from solids contained in slurry sludges, etc., is the one provided by U.S. Pat. No. 4,752,402. This method forces a liquid-permeable fabric downward into a slurry so that solid materials are retained below the liquid-permeable fabric and liquids may be pumped or drained from above the liquid-permeable fabric.
Other processes such as those disclosed by U.S. Pat. Nos. 4,593,760, 4,660,639 and 4,730,672 involve the injection of air into a vadose zone to urge the contaminants toward a withdrawal well.
A process for removing contaminants from an aquifer itself is shown in U.S. Pat. Nos. 4,183,407 and 4,809,673. The process disclosed in the latter teaches the injection of atmospheric air into the aquifer to urge the contaminant out of the aquifer and into the vadose zone toward a withdrawal well. This process is commonly referred to as sparging. One major drawback to sparging is the stimulation of bacterial growth or the formation of inorganic precipitates in the aquifer saturated zone or by injection of oxygen thereinto.
Still another process for removing and disposing of or neutralizing volatile contaminants in a below ground aquifer is shown by U.S. Pat. No. 4,945,988. The process includes the injection of substantially oxygen free air into the aquifer to retard the formation of aerobic bacteria and the injection of oxygen rich air into the vadose zone to stimulate bacterial growth and increase the recovery of contaminants. The volatilized contaminants are extracted from the soil by means of withdrawal wells that terminate in the vadose zone.
The Environmental Protection Agency (EPA) has targeted over a thousand sites for Superfund cleanup. Yet, up to the present time only a fraction of the sites are being partially cleaned up. According to the U.S. News & World Report, the cost, so far, has been a staggering $4 billion.
Accordingly, there is still a need for an effective method for in situ decontaminating soil that is simple and fast while at the same time being capable of reducing the level of contamination to an environmentally acceptable value.